1. Field of the Invention
The present invention relates to random access storage devices and, more particularly, to such devices using flexible magnetic disks with noncontact magnetic heads for reading and/or writing magnetic data on a selected disk. Still, more particularly, the invention is directed to the head arm assembly which supports and positions the magnetic head relative to the oxide surface of the selected disk.
2. Prior Art
The use of a head arm assembly for supporting and positioning a magnetic head so as to access data from a track on a selected disk of a multi-disk rotating mass storage system is well known in the prior art. Conventional mass storage systems consist of a stack of rotating member disks and an accessing apparatus. The stack of disks is usually fixed or clamped at the midpoint to a tubular member. The tubular member and disks are mounted to the rotating shaft of a motor for rotation therewith. The motor is usually mounted to a support frame. At least one reference plate is securely mounted on the motor shaft on the tubular member for rotation with the stack of disks. The reference plate is positioned to be in axial alignment with the accessing apparatus when said apparatus is in an initial or home position.
The accessing apparatus includes a positioning apparatus which may be a voice coil motor, a linear actuator, or a linear motor. The positioning apparatus rides on a member of rail which is parallel to the axis of rotation of the disk file. The positioning apparatus positions the magnetic head axially so as to access any selected disk in the disk file. A radial member or carriage which positions the magnetic head radially so as to access any desired track on the selected disk. The radial member is fitted with a head arm support. The magnetic transducer or head is seated on the head arm support. The direction of travel of the carriage and its attachment (i.e., the head arm support and magnetic head) is orthogonal to the axis of rotation of the disk file. The orientation between the rotating disk file and the associated accessing apparatus may be vertical or horizontal. In order to allow access into the disk pack convention means, for example, vacuum or partitioning blades are used to open the pack between two selected disks. The magnetic head is then inserted in the opening and transduces (i.e., reads and/or writes) data on a selected track on one of the two disks. A more detailed discussion of the aforementioned type of flexible disk storage system is described in U.S. Pat. No. 3,940,794 issued to Donald E. Griffiths et al and assigned to the assignee of the present invention.
In order to improve the accessibility and reliability of conventional random access flexible disk devices, several attempts have been made, in the prior art, to improve the head arm support or the mechanism which splits and partitions the pack.
In one attempt a random access memory with ultra thin rotating flexible magnetic disks is stabilized while being partitioned by an air foil blade. The air foil blade is a hollow structure with a contour of varying cross-sectional areas extending from the tip which is first inserted into a pack to the opposite end which is mounted to a carriage assembly. Due to the varying contour of the blade, it operates as an air foil. By thrusting the blade into the rotating disks at a selected interface, the disks are separated into discretely rotating segments which diverge and converge about the blade to form a sizeable opening suitable for transducing access. Pressurized air which is emitted from the air foil blade further stabilizes the opening. A more detailed description of the aforementioned blade is given in U.S. Pat. No. 3,852,820.
In still another attempt, the magnetic transducer of a flexible disk file is mounted on an access arm which is blunt on one side edge and sharp on the other side edge and increases in thickness gradually from the sharp edge to the blunt edges where the maximum thickness occurs. A magnetic transducer is positioned adjacent the blunt edge and in from the tip of the access arm. When the arm is inserted into the flexible disk file with the disk rotating in a direction to transverse the access arm from the sharp edge towards the blunt edge, the transducing interface between the transducer and the rotating disk is relatively close. Stated another way, the disk file is relatively close to the transducer. A more detailed discussion of the aforementioned blade is given in U.S. Pat. No. 3,975,769.
Although the above-described devices have improved accessibility to flexible disk files, these devices do not address the fail-safe aspect of a random access flexible disk storage system. Failsafe, as used herein, means that if an unusual condition or problem is generated during system operation then the splitting mechanism or the head arm support will operate so as to minimize damage to the disk and/or data loss. There are several problem conditions which can develop during system operation which will result in damage to disk storage systems. Probably one common problem condition is power loss. The power loss condition is particularly devastating when the head is in the disk pack simultaneously with the power loss. As is well known to those skilled in the art, the opening in a flexible disk pack which allows a magnetic transducer to enter so as to transduce data from a selected track on a target disk, is generated and controlled by the rotational speed of the pack and/or air jets from an air supply source. Generally the air jets which may be internal to the pack, are used for splitting the pack. When a flexible disk pack is rotating at its normal operational velocity, the surface of a target disk which interfaces the opening is relatively flat (that is free from waves and undulations). However, as the disk pack slows down, bow waves are generated in the surface of the target disk. The bow waves have the potential to interact with the head arm support and/or the transducer. Due to the fact that the disks are fragile, the interaction results in the head and/or head support arm tearing the disk.
In the situation where air pressure is lost, the pack will close on the head and head arm support. This condition also results in damage to the disk pack and loss of recorded information.
Another condition which results in damage to the disks occurs when the head arm support and transducer remain in a closed pack for a relatively long period of time. Under normal operating conditions the media or disk is flying relative to the head and head arm support. However, as the pack closes the head and head arm support are in physical contact with the surface of the disk. This condition generates heat and wear which damage the disk.
Another source of damage to a disk pack occurs when the head arm support and head are inserted into a closed pack or partially opened pack. Generally, the insertion and withdrawal of the head arm support is done by a positioning mechanism. The positioning mechanism is controlled by a servo loop. Controlled signals are generated which instruct the mechanism as to when to insert the head arm support. It is not unusual to have a control signal issued to the mechanism when in fact the disk pack is not opened. The force with which the head arm impacts the disks in the pack results in tearing the disk.
Another type of problem which plagues the prior art head arm support mechanism is that of resonance. This problem is particularly associated with long stroke actuators positioning relatively long head arm support mechanisms. "Long stroke" as it is used herein, means an actuator in which the maximum effective length over which the carriage assembly is moved is greater than two inches. Likewise, a relatively long head arm support means a head arm having a length not less than two inches. The problem is further augmented when the long stroke actuator is used with a high performance, flexible disk storage system. As is well known to one skilled in the art, the rotational speed of the disk coupled with the space on the disk reserved for servo information, dictates the data throughput for the flexible disk file storage system. In order to maintain a relatively high data throughput, the actuator has to move the head arm at a relatively high speed so as to access data from a selected disk in the file. The rapid to and fro motion coupled with the long stroke and the long support arm tends to aggravate the resonance problem.
As the arm resonates, the resonance is transferred to the transducer which it supports. This results in head bounce and roll in the transducer. Head bounce causes variation in the signal recorded on the selected disk or the signal outputted from the transducer. If the output signal is analog, the amplitude of the signal is generally affected. When the output signal is digital, the phase of the signal is affected. Changes in the signal, be they in the amplitude or in the phase, affect the triggering level of the electrical circuits which process the signal. Generally, the electrical circuits needed to process varying signals are more costly and more complicated.
The resonance phenomenon further introduces instability in the servo loop which controls the head arm support. As is well known to those skilled in the art, the servo loop allows the transducer to faithfully follow a selected track on a target disk in the disk storage system. Instability results in servo error which reduces system throughput. It has been observed that whenever the head support arm resonates at a frequency sufficiently close to the cross-over point where the gain of the servo loop is unity, the instability and its associated effect is mostly predominant.